Consul Official Image Build

We start with an Alpine base image and add CA certificates in order to reachthe HashiCorp releases server. These are useful to leave in the image so thatthe container can access Atlas features as well.

Official HashiCorp builds of some base utilities are then included in theimage by pulling a release of docker-base. This includes dumb-init and gosu.See https://github.com/hashicorp/docker-base for more details.

Finally a specific Consul build is fetched and the rest of the Consul-specificconfiguration happens according to the Dockerfile.